Chemistry 9701 · AS & A Level · Chemistry of transition elements

Chemistry of transition elements — practice question

The blue colour of an aqueous copper(II) sulfate solution arises from the presence of $[\text{Cu(H}_2\text{O)}_6]^{2+}$ complex ions.
(a(i))[1]

Write an equation for the reaction of $[\text{Cu(H}_2\text{O)}_6]^{2+}$ ions with $\text{NaOH(aq)}$.

(a(ii))[1]

Write an equation for the reaction of $[\text{Cu(H}_2\text{O)}_6]^{2+}$ ions with excess conc. $\text{HCl}$.

(b(i))[1]

Two stereoisomers can exist for the formula $[\text{Cu(NH}_3)_4(\text{H}_2\text{O})_2]^{2+}$. Complete the boxes below so that both stereoisomers are shown.

(b(ii))[1]

Use your answer in (b)(i) to work out whether each of these isomers is polar or non-polar.\nPolarity of isomer 1.\nPolarity of isomer 2.

(c(i))[1]

Define the stability constant.

(c(ii))[1]

Compare the stabilities of the $[\text{Cu(H}_2\text{O)}_6]^{2+}$ and $[\text{Cu(NH}_3)_4(\text{H}_2\text{O})_2]^{2+}$ complex ions. Give a reason for your answer.

(c(iii))[2]

Write an expression for the stability constant, $K_{\text{stab}}$, for the $[\text{Cu(NH}_3)_4(\text{H}_2\text{O})_2]^{2+}$ complex ion. State the units of the stability constant.

(c(iv))[1]

In one solution, the concentration of the $[\text{Cu(NH}_3)_4(\text{H}_2\text{O})_2]^{2+}$ complex ion is $0.0074\ \text{mol dm}^{-3}$ and the concentration of $\text{NH}_3$ is $0.57\ \text{mol dm}^{-3}$. Use your expression in (c)(iii) together with $K_{\text{stab}} = 1.40 \times 10^{13}$ to work out the concentration of the $[\text{Cu(H}_2\text{O)}_6]^{2+}$ complex ion in this solution.

(d)[2]

Phenanthroline, $\text{C}_{12}\text{H}_8\text{N}_2$, and ethanedioate ions, $\text{C}_2\text{O}_4^{2-}$, are bidentate ligands. Ruthenium(III) ions, $\text{Ru}^{3+}$, form an octahedral complex with phenanthroline and chloride ions. The complex ion includes two phenanthroline molecules. Iron(III) ions, $\text{Fe}^{3+}$, form an octahedral complex made only with ethanedioate ions. Deduce the formula and charge of each of these complex ions.\n$\text{Ru}^{3+}$ complex.\n$\text{Fe}^{3+}$ complex.

Worked solution & mark scheme

This 11-mark question has a full step-by-step worked solution and mark scheme. One marking point: $\mathrm{[Cu(H_2O)_6]^{2+} + 2OH^- \to [Cu(OH)_2(H_2O)_4] + 2H_2O}$

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